Liquid fuel injector and injector system for a small gas turbine engine

ABSTRACT

The liquid fuel injector includes continuous air assist with both the liquid fuel injector tube and the surrounding air assist tube constrained or reduced in diameter at the discharge ends thereof. The associated gas turbine engine compressor provides combustion air to the combustor and also air for continuous air assist with a separate air pump. The air assist air may be cooled in a heat exchanger either before or after the separate air pump. The liquid fuel injector may be mounted within an annular air blast tube and directed tangentially at the inner wall of the combustor. 
     A solenoid valve controlled purge line, together with solenoid valves in the liquid fuel and air assist lines can be utilized to purge any liquid fuel in the liquid fuel injector and manifold upon shutdown of the combustor. The liquid fuel injector can also be insulated and liquid fuel can be used to cool the fuel injector.

This application is a division of application Ser. No. 08/864,279, filedMay 28, 1997, now U.S. Pat. No. 5,966,426.

TECHNICAL FIELD

This invention relates to the general field of fuel injectors and moreparticularly to an improved liquid fuel injector and system for a smallgas turbine engine.

BACKGROUND OF THE INVENTION

In a gas turbine engine, inlet air is continuously compressed, mixedwith liquid fuel in an inflammable proportion, and then contacted withan ignition source to ignite the mixture which will then continue toburn. The heat energy thus released then flows in the combustion gasesto a turbine where it is expanded and converted to rotary energy fordriving equipment such an electrical generator. The combustion gases arethen exhausted to atmosphere after giving up some of the remaining heatto the incoming air provided from the compressor.

As is well known, gas turbine engines typically include a rotor and aturbine wheel rotatable about a generally horizontal axis. Notinfrequently, an annular combustor surrounds this horizontal axis and isprovided with a plurality of angularly spaced liquid fuel injectorswhereby liquid fuel is injected into the combustor to be ignited andburned and the combustion products ultimately directed at the turbinewheel to spin the turbine wheel. At a location that is usually externalof the combustor, a ring-like manifold is utilized as a liquid fuelmanifold to interconnect the various annularly spaced liquid fuelinjectors.

Because the rotational axis of the compressor and turbine wheel ishorizontal, this ring-like manifold would normally be in a verticalplane. This in turn means that the pressure acting on the liquid fuel atthe lowermost liquid fuel injector will be greater than the pressureacting on the liquid fuel at the highest injector. The pressuredifference is a consequence of gravity on the vertical column of liquidfuel in the manifold and thus is generally referred to as "manifoldhead".

While in a larger gas turbine this may not represent a significantproblem, in a small gas turbine with nominally small liquid fuel flows,substantial nonuniformity in liquid fuel injection may be produced. Thisin turn can lead to the development of hot spots within the small gasturbine engine combustor which can shorten its life as well as reduceoperating efficiencies because of poor localized combustion. Achievinguniform turbine inlet temperature distribution minimizes hot spots andcold spots to maximize efficiency of operation as well as to prolong thelife of the turbine parts exposed to the hot combustion gases.

While a simple solution might be to provide a large number of liquidfuel injectors to insure that the liquid fuel is uniformly distributedto the combustion air, the number of liquid fuel injectors not onlyincrease costs, but also means that each individual liquid fuel injectorwould be smaller when the overall liquid fuel consumption remains thesame.

Also, while each liquid fuel injector can theoretically be provided withan individual orifice, this requires an increase in liquid fuel pressurein order to deliver liquid fuel past the orifice into the combustionchamber. As a consequence, in order to have substantially uniform liquidfuel injection at all injector locations, the manifold head pressure atthe lowermost liquid fuel injector would be relatively small compared tothe pressure applied to the liquid fuel at all other orifices. In orderto increase this pressure drop at each liquid fuel injector location,the orifices must be made to be relatively small. As a consequence,these small orifices would be prone to clogging. Once an orifice isclogged and the corresponding liquid fuel orifice is blocked, theproblem of hot spots returns.

The design of combustion systems for small gas turbines is hardly asimple scale down of designs that are operative in large gas turbineengines. Regardless of combustor size, there is a minimum residence timefor liquid fuel and air within the combustor necessary to effectsufficiently complete combustion to generate the gases to drive aturbine wheel. Given the dynamics of gas flow in and out of a combustorto a turbine wheel, it should be readily apparent that as the size ofthe combustor is decreased, conventional techniques would only bestarting the combustion process, if it occurred at all, as the air andliquid fuel mixture was exiting the combustor outlet.

Moreover, in small combustors, which necessarily are provided with smallliquid fuel injectors and consequently have relatively small liquid fuelflow at each injector, it is difficult to provide the needed fine liquidfuel atomization with conventional techniques. This is primarily due tothe fact that the small scale effects increased viscous losses resultingin a deterioration in liquid fuel atomization at the injector. Inaddition, the small liquid fuel metering orifices associated with suchsmall liquid fuel injectors tend to promote premature liquid fuel spraydeterioration due to orifice fouling which in turn can cause earlyengine failure due to gas temperature maldistributions. Conventionalliquid fuel injector design techniques are already ordinarily complexand costly. When, however, they are employed to reduced scale design foruse in small combustors, the complexity and cost becomes prohibitive.

Recognizing these difficulties, in recent years there has been adefinite trend towards combustor systems in which the path of travel forthe liquid fuel and air in the flame zone, as well as the products ofcombustion, are in the circumferential direction rather than in theaxial direction as in a conventional combustion system. These annularcombustors employ a technique called "sidewinding" to minimize the axialflow components of liquid fuel, air and products of combustion. Thisarrangement maximizes the time available for combustion within a givensmall volume and also permits a significant reduction in the number ofliquid fuel injectors without the resultant undesirable high turbineinlet temperature maldistributions as would be obtained usingconventional design techniques if the number of liquid fuel injectors isreduced. Maximizing the time available for mixing and combustion whileminimizing the number of liquid fuel injectors is most advantageous fromcost and efficiency standpoints, particularly when accomplished in smallgas turbines.

In recent annular combustors operating on the sidewinder technique, itis typical to have a plurality of air blast and/or air assist tubescircumferentially spaced about the combustor and normally located in theradially outer wall thereof. While air blast and air assist are somewhatsimilar, air blast generally has a higher velocity and is hotter thanair assist. One end of each tube is open to the interior of thecombustor while the opposite end is opened to the space between theradially outer wall of the combustor and the outer combustor case. As isknown, this space is typically charged with compressed air from thecompressor associated with the gas turbine engine. These tubes aredirected tangentially into the annular combustion space of thecombustor.

For liquid fuel injection purposes, liquid fuel injector tubes havetypically been mounted within the air assist tubes and as a consequenceliquid fuel atomization of liquid fuel injected from the tubes may beachieved as the liquid fuel is injected toward the combustion space inan associated air assist tube as the air passing though the air assisttube provides air atomization. As smaller and smaller combustors aredesigned, however, the diameter of the air assist tubes becomescommensurately reduced to the point where it is difficult to place theliquid fuel injector tubes inside the air assist tube.

Also, since the liquid fuel injector orifices or outlets are within thecombustor, they are exposed to substantial heat. During normaloperations, this does not present a problem since the flow of liquidfuel through the liquid fuel injector provides a cooling effect.Further, the propagation of combustion along with the flow of air servesto prevent undesirable overheating of the liquid fuel injectors. Once,however, operation ceases, neither liquid fuel nor air flows through theliquid fuel injector. Consequently, residual heat in the combustor areawill cause elevation of the temperature of the liquid fuel injectors. Interms of the materials of which the liquid fuel injectors areconstructed, this raising in temperature upon cessation of operationdoes not present a problem. The presence, however, of residual liquidfuel in the liquid fuel injector at such time will frequently cause acoking problem. Being carbonaceous in nature, such liquid fuel, uponbeing heated will begin to undergo a destructive distillation reactionand a coke-like and/or tarry residue will remain. Such a residue willquickly clog the liquid fuel injector and will result in improperoperation during subsequent start-up.

SUMMARY OF THE INVENTION

In the present invention, the improved liquid fuel injector is providedwith continuous air assist with both the liquid fuel injector tube andthe surrounding air assist tube constrained or reduced in diameter atthe discharge end thereof. The associated compressor provides combustionair to the combustor and air for continuous air assist with a separateair pump. The air assist air can be cooled in a heat exchanger eitherbefore, after, or before and after the separate air pump. The improvedliquid fuel injector can be mounted within an annular air blast tube anddirected at the inner wall of the combustor.

A solenoid valve controlled purge line, together with solenoid valves inthe liquid fuel and air assist lines can be utilized to purge any liquidfuel in the liquid fuel injector and manifold upon shutdown of thecombustor. The liquid fuel injector can also be insulated and liquidfuel can be used to cool the fuel injector.

BRIEF DESCRIPTION OF THE DRAWINGS

Having thus described the present invention in general terms, referencewill now be made to the accompanying drawings in which:

FIG. 1 is a perspective view, partially cut away, of a turbogeneratorutilizing the improved liquid fuel injector system of the presentinvention;

FIG. 2 is a sectional view of the improved liquid fuel injector of thepresent invention;

FIG. 3 is an end view of an alternate improved liquid fuel injector ofthe present invention;

FIG. 4 is a sectional view of the alternate improved liquid fuelinjector of FIG. 3 taken along line 4--4 of FIG. 3,

FIG. 5 is a block diagram, partially schematic view of a continuous airassist cooling system for the improved liquid fuel injector of FIGS.2-4;

FIG. 6 is a block diagram, partially schematic view of an alternatecontinuous air assist cooling system for the improved liquid fuelinjector of FIGS. 2-4;

FIG. 7 is a sectional view of the improved liquid fuel injector of thepresent invention mounted in the combustor casing;

FIG. 8 is a cross sectional view of the mounted improved liquid fuelinjector of FIG. 7 taken along line 8--8;

FIG. 9 is a schematic view of an annular combustor having the improvedliquid fuel injector of FIGS. 2-8;

FIG. 10 is a cross sectional view of the annular combustor of FIG. 9taken along line 10--10;

FIG. 11 is a schematic view of a liquid fuel purge system for improvedliquid fuel injector of FIGS. 2-10;

FIG. 12 is a schematic view of an alternate liquid fuel purge system forthe improved liquid fuel injector of FIGS. 2-10; and

FIG. 13 is a schematic view of an insulated improved liquid fuelinjector of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A turbogenerator 12 utilizing the liquid fuel injector system of thepresent invention is illustrated in FIG. 1. The turbogenerator 12generally comprises a permanent magnet generator 20, a power head 21, acombustor 22 and a recuperator (or heat exchanger) 23.

The permanent magnet generator 20 includes a permanent magnet rotor orsleeve 26, having a permanent magnet disposed therein, rotatablysupported within a permanent magnet generator stator 27 by a pair ofspaced journal bearings. Radial permanent magnet generator statorcooling fins 28 are enclosed in an outer cylindrical sleeve 29 to forman annular air flow passage which cools the permanent magnet generatorstator 27 and thereby preheats the air passing through on its way to thepower head 21.

The power head 21 of the turbogenerator 12 includes compressor 30,turbine 31, and bearing rotor 32 through which the tie rod 33 to thepermanent magnet rotor 26 passes. The compressor 30, having compressorimpeller or wheel 34 which receives preheated air from the annular airflow passage in cylindrical sleeve 29 around the permanent magnetgenerator stator 27, is driven by the turbine 31 having turbine wheel 35which receives heated exhaust gases from the combustor 22 supplied withpreheated air from recuperator 23. The compressor wheel 34 and turbinewheel 35 are supported on a bearing shaft or rotor 32 having a radiallyextending bearing rotor thrust disk 36. The bearing rotor 32 isrotatably supported by a single journal bearing within the centerbearing housing 37 while the bearing rotor thrust disk 36 at thecompressor end of the bearing rotor 32 is rotatably supported by abilateral thrust bearing.

Intake air is drawn through the permanent magnet generator 20 by thecompressor 30 which increases the pressure of the air and forces it intothe recuperator 23. In the recuperator 23, exhaust heat from the turbine31 is used to preheat the air before it enters the combustor 22 wherethe preheated air is mixed with liquid fuel and burned. The combustiongases are then expanded in the turbine 31 which drives the compressor 30and the permanent magnet rotor 26 of the permanent magnet generator 20which is mounted on the same shaft as the turbine 31. The expandedturbine exhaust gases are then passed through the recuperator 23 beforebeing discharged from the turbogenerator 12.

The turbogenerator 12 is able to operate on whatever liquid fuel isavailable. Ignition of the liquid fuel from the liquid fuel injectors 14produces heat and the liquid fuel flow is sustained and accelerates theturbogenerator which raises the pressure of the turbogeneratorcompressor 30. As the turbogenerator compressor 30 increases thepressure of the combustion air, the liquid fuel pressure must becorrespondingly increased to keep it somewhat higher so that there is apositive flow; of liquid fuel to the combustor injectors.

The liquid fuel injectors 14 of the present invention are illustrated inmore detail in FIGS. 2-4. As generally shown in FIG. 1, the combustor 22includes a plurality of liquid fuel injectors 14 spaced around theannular periphery thereof.

The liquid fuel pressure at the lowest liquid fuel injector 14 will behigher than the liquid fuel pressure at the highest liquid fuel injector14 by an amount "" times "h", where "" is liquid fuel density and "h" isthe vertical distance between the lowest liquid fuel injector 14 and thehighest liquid fuel injector 14. In order to compensate for thismanifold head effect, the liquid fuel injector tube 16 can be swageddown to a smaller diameter discharge end 17. In addition, theconcentrically arranged air assist tube 18 can also be swaged downcorrespondingly to a smaller diameter discharge end 19. This willproduce a liquid fuel pressure drop which only partly compensates formanifold head. The application of liquid fuel back pressure isparticularly advantageous in reducing vapor lock when volatile liquidfuels such as gasoline are utilized.

Without the swaged smaller diameter discharge end 17 on the liquid fuelinjector tube 16, orifices might otherwise have to be utilized in theliquid fuel injector tube 16 to take a significant pressure drop suchthat the liquid fuel pressure times "h" would create only minor liquidfuel maldistributions. In a small gas turbine engine, the resultingorifice would be very small, costly and prone to plugging up and liquidfuel pressure would be very high which could cause cost and reliabilityproblems. For example, in a small gas turbine on the order of 24 kW witha manifold head of 8 inches, the orifice size would be only 0.003 inchesand the maximum liquid fuel pressure at fuel power would be a very high1500 psi.

The air assist air pressure in the improved liquid fuel injector 14would be on the order of several psi and would accelerate the liquidfuel jet from the liquid fuel injector tube 16. This would also serve tocompensate for the manifold head effect. At the same time liquid fuelpressure would be relatively low and the liquid fuel injector tubedischarge end opening would be relatively large when compared to aninternal orifice.

As illustrated in FIGS. 5 and 6, a small air pump 38 driven at more orless constant speed by one of a variety of means such as an electricmotor, gear box, etc., (not shown) can be provided. The air pump 38 mostconveniently can use compressed air tapped off of the gas turbinecompressor 30. This will minimize the size of the air pump 38 and itspower requirements. FIG. 5 shows the air assist air cooled by a liquidfuel or air heat exchanger 40 before the air is supplied to the liquidfuel injectors 14. As an alternative shown in FIG. 6, the air assist airmay be cooled in a liquid fuel or air heat exchanger 42 before the airpump 28.

Cooling the air assist air is advantageous in helping to cool the liquidfuel injectors 14. It should be appreciated that in a small annularcombustor, particularly of the recuperated type, the liquid fuel flowwould, at no load, be extremely low. The problem is made worse since theliquid fuel needs to be shared amongst several liquid fuel injectors 14.There simply is no practical liquid fuel injector that cansatisfactorily atomize the liquid fuel at these low liquid fuel flowrates, for example, on the order of 21/2 pounds per hour at no load. Bykeeping the air pump in continuous operation, the liquid fuel can bewell atomized by a simple means.

Combustion of liquid fuels is highly dependent on effective atomizationto achieve high rates of mixing and evaporation. Decreasing liquid fueldroplet size out of the liquid fuel injectors 14 will result in fasterevaporation which translates into easier ignition, a wider burningrange, improved mixing, and reduced pollutant emissions.

Moreover, a recuperated gas turbine engine has a vast range of operatingconditions. Thus, in a cold start the air temperature entering thecombustor, at full load, could be quite low, say around 400 degreesFahrenheit, but when filly warmed up the air temperature can be as highas 1,100 degrees Fahrenheit. For a typical recuperated small gas turbineengine, the respective liquid fuel flows could be greater than 38 poundsper hour to about 151/2 pounds per hour, respectively. With perhaps adesign point hot pressure drop of 3%, then the cold design pointpressure drop would be only about 1.6%. Hence when operating cold, thepressure drop would not be sufficient for satisfactory liquid fuelatomization if conventional liquid fuel atomizers were employed.Continuous air assist from the air pump 38 can solve this problem.

As shown in FIGS. 7 and 8, the liquid fuel injectors 14 can be mountedthrough flange 43 in the combustor case 44 and slidably mounted in anair blast tube 47 which extends through the outer combustor case 46. Thesupport sleeve 49 in the spider support 48 of the air blast tube 47provides a sliding fit for the liquid fuel injector 14. The liquid fuelinjector 14 is mounted somewhat upstream of the exit orifice 50 of theair blast tube 47 in a strongly accelerating airflow. Hence there are nodamaging wakes from the liquid fuel injector 14 that might otherwiseenter the combustor 22. The combustor pressure drop derives air fromoutside the combustor 22, through the air blast tube exit orifice 50 ata high air velocity of several hundred feet per second. Thus, this airis thoroughly mixed with the well atomized liquid fuel/assist airmixture which is most advantageous for good combustion.

FIGS. 9 and 10 illustrate the liquid fuel injectors 14 mounted in nearproximity to the combustor dome 51 and directed so that the liquidfuel/air jets emanating from these liquid fuel injectors 14 are directedto flow more or less tangentially towards the combustor inner wall 52and thus aiming at the next liquid fuel injector 14. It is verydesirable to minimize the number of liquid fuel injectors and the abovearrangement enables this to be accomplished.

In this instance, the combustor is made considerable larger than wouldotherwise be common practice. A common sizing measure for a combustor isBTU per cubic feet hour atmosphere. A typical combustor might achieve avalue of 6 times 10⁶ or more. The combustors of the present inventionhave a value of 0.6 times 10⁶ or even less, i.e. a factor of ten timeslarger than a combustor with conventional practice. By this means thecombustor of FIGS. 9 and 10 can operate most satisfactorily while usingonly 3 liquid fuel injectors which is a considerable advance over thecurrent practice.

FIGS. 11 and 12 illustrate the combustor of FIGS. 9 and 10 with a liquidfuel injector purge system. In FIG. 11, the liquid fuel line includes aliquid fuel control valve 57 and a liquid fuel on/off solenoid valve 56.The air assist line or conduit (shown as a dashed line) includes an airassist solenoid valve 58. In addition, a purge line 53 with a purge linesolenoid valve 54 is provided from the liquid fuel line downstream ofthe liquid fuel on/off solenoid valve 56.

At engine shutdown, the air pressure inside the combustor wouldordinarily, depending upon engine speed and load, be from 29 psia andupwards. In order to purge the liquid fuel injectors 14, the purge linesolenoid valve 54 is opened and both the liquid fuel on/off solenoidvalve 56 and air assist solenoid valve 58 are closed. The back pressurefrom the circa 29 psi pressure in the combustor will back flow theliquid fuel out of the liquid fuel manifold and overboard through theliquid fuel purge line 53. It is critical that the air assist solenoidvalve be off to prevent the liquid fuel in the liquid fuel manifold frombeing sucked into the gas turbine engine at an uncontrolled rate whichcould cause damaging explosions.

Alternately, as shown in FIG. 12, a restrictive orifice 59 can beincluded in the purge line 53 after the purge line solenoid valve 54 andthe air assist solenoid valve can be eliminated. In this instance, withthe liquid fuel solenoid valve 56 closed and the purge line solenoidvalve 54 open, the liquid fuel in the manifold will be sucked out at acontrolled rate by virtue of the restrictive orifice 59 in the liquidfuel purge line 53.

The liquid fuel injector system of FIG. 13 is designed to cool theliquid fuel and thus prevent vapor lock in the combustor when a liquidfuel such as gasoline is utilized. The liquid fuel injector tube 62 caninclude a liquid fuel spill orifice 66 near the smaller diameterdischarge end 64 to permit excess liquid fuel to spill into a concentricspill tube 68 which also serves as the inner wall of the air assist tube70. The exterior of the air assist tube 70 is insulated with insulation72. The air assist inlet 75 to the air assist tube 70 is provided in thesheet metal support 76 which serves to thermally isolate the liquid fuelinjector 14.

The improved liquid fuel injector and system described above can achievesignificant advantages over conventional liquid fuel injectors andsystems. Not only are the number of individual liquid fuel injectorsminimized but their construction and placement result in improvedperformance and eliminate many of the difficulties with prior systems.

While specific embodiments of the invention have been illustrated anddescribed, it is to be understood that these are provided by way ofexample only and that the invention is not to be construed as beinglimited thereto but only by the proper scope of the following claims.

What we claim is:
 1. A liquid fuel injector purge system for a gas turbine engine, comprising:an annular combustor having an outer wall, an inner wall, a closed upstream end, and an open discharge end; a plurality of liquid fuel injectors spaced around the periphery of the closed end of said combustor to direct liquid fuel/air into said combustor; a liquid fuel inlet conduit to each of said plurality of liquid fuel injectors, said liquid fuel inlet conduit having a liquid fuel control valve and a liquid fuel on/off solenoid valve; an air assist conduit to each of said plurality of liquid fuel injectors, and a liquid fuel purge conduit extending from said liquid fuel conduit, disposed between said liquid fuel on/off solenoid valve and said plurality of liquid fuel injectors, and having a purge line solenoid valve and a restrictive orifice, so that when said gas turbine engine is shut down and said liquid fuel inlet or/off solenoid valve is closed, liquid fuel is sucked through the liquid fuel purge line restrictive orifice with the liquid fuel purge line solenoid valve open.
 2. The liquid fuel injector purge system of claim 1 wherein each of said liquid fuel injectors generally directs fuel/air tangentially towards the inner wall of said combustor.
 3. The liquid fuel injector purge system of claim 1 wherein each of said plurality of liquid fuel injectors comprise:a cylindrical liquid fuel tube having a reduced diameter discharge end; and a cylindrical air assist tube concentrically disposed around said cylindrical liquid fuel tube and having a reduced diameter discharge end disposed around the reduced diameter discharge end of said cylindrical liquid fuel tube.
 4. The liquid fuel injector purge system of claim 3 wherein the exterior of said cylindrical air assist tube is insulated.
 5. The liquid fuel injector purge system of claim 3 wherein each of said plurality of liquid fuel injectors additionally includes a cylindrical air blast tube disposed around the reduced diameter discharge ends of said cylindrical liquid fuel tube and said cylindrical air assist tube and having a plurality of radial spider supports extending from said air blast tube to support the reduced diameter discharge end of said cylindrical air assist tube.
 6. The liquid fuel injector purge system of claim 3 wherein said air assist conduit to continuously deliver compressed air from the gas turbine engine compressor to said cylindrical air assist tube during operation of said liquid fuel combustion system includes a pump disposed in said air assist conduit to raise the pressure of the compressed air delivered to said air assist tube.
 7. The liquid fuel injector purge system of claim 6 and in addition a heat exchanger in said air assist conduit upstream of said pump to remove heat from said compressed air.
 8. The liquid fuel injector purge system of claim 6 and in addition a heat exchanger in said air assist conduit downstream of said pump to remove heat from said pumped compressed air.
 9. A liquid fuel injector purge system for a gas turbine engine, comprising:an annular combustor having an outer wall, an inner wall, a closed upstream end, and an open discharge end; a plurality of liquid fuel injectors spaced around the periphery of the closed end of said combustor to direct liquid fuel/air tangentially towards the inner wall of said combustor, each of said plurality of liquid fuel injectors including a cylindrical liquid fuel tube having a reduced diameter discharge end, a cylindrical air assist tube concentrically disposed around said cylindrical liquid fuel tube and having a reduced diameter discharge end disposed around the reduced diameter discharge end of said cylindrical liquid fuel tube, and a cylindrical air blast tube disposed around the reduced diameter discharge ends of said cylindrical liquid fuel tube and said cylindrical air assist tube and having a plurality of radial spider supports extending from said air blast tube to support the reduced diameter discharge end of said cylindrical air assist tube; a liquid fuel inlet conduit to each of said plurality of liquid fuel injectors, said liquid fuel inlet conduit having a liquid fuel control valve and a liquid fuel on/off solenoid valve; an air assist conduit to each of said plurality of liquid fuel injectors; and a liquid fuel purge conduit extending from said liquid fuel conduit, disposed between said liquid fuel on/off solenoid valve and said plurality of liquid fuel injectors, and having a purge line solenoid valve and a restrictive orifice, so that when said gas turbine engine is shut down and said liquid fuel inlet on/off solenoid valve is closed, liquid fuel is purged through said liquid fuel purge line restrictive orifice with the liquid fuel line solenoid valve open.
 10. The liquid fuel injector purge system of claim 9 wherein said air assist conduit to continuously deliver compressed air from the gas turbine engine compressor to said cylindrical air assist tube during operation of said liquid fuel combustion system includes a pump disposed in said conduit to raise the pressure of the compressed air delivered to said air assist tube.
 11. The liquid fuel injector purge system of claim 10 and in addition a heat exchanger in said air assist conduit upstream of said pump to remove heat from said compressed air.
 12. The liquid fuel injector purge system of claim 10 and in addition a heat exchanger in said air assist conduit downstream of said pump to remove heat from said compressed air. 